The present invention relates to a new and improved construction of a displacement machine -- also referred to as a positive displacement machine-- for compressible media.
Generally speaking, the positive displacement machine of the invention is of the type incorporating a displacement compartment bounded by a spiral-shaped inwardly situated side wall and a spiral-shaped outwardly situated side wall, the displacement compartment describing a span angle exceeding 360.degree. and extending between an inlet and an outlet. Further, there is provided for the displacement compartment a displacement element arranged therein and carrying out a circulatory movement relative thereto, this displacement element likewise possessing the shape of a spiral and having practically the same span angle as the displacement compartment. The displacement element, during the course of the circulatory movement, always contacts both the outer situated side wall and also the inner situated side wall at least at one respective progressing or advancing contact line.
During operation of positive displacement machines of this type there is bounded, on the one hand, by the displacement element and, on the other hand, by the one side wall of the displacment compartment a conveying compartment or chamber which, during the course of the circulatory or gyrating movement migrates along the spiral and accordingly changes its volume. As a result, depending upon whether the migratory movement occurs along the spiral from the outside towards the inside or from the inside towards the outside, there results a compression or expansion, respectively, of the conveyed medium.
An approximation of the compressibility factor or expansion factor, as the case may be, can be derived from the ratio between the mean or average diameter of the section of the spiral of the displacement compartment which spans 360.degree. and following the inlet of the machine and the mean or average diameter of the section of the spiral of the displacement compartment which spans 360.degree. and directly precedes the machine outlet. There is equated to the mean diameters the arithmetic mean between the mean inner diameter of the outer situated side wall of the displacement compartment and the mean outer diameter of the inner situated side wall of the displacement compartment.
The conveying capacity of such machine furthermore is dependent, among other things, upon the spacing between the outer situated side wall and the inner situated side wall which is automatically constant over the entire course of the displacement compartment due to the circulatory movement of the displacement element, i.e. viewed in the conveying direction upon the "width" of the displacement compartment. This spacing or width, on the one hand, corresponds to one-half of the difference between the inner diameter of the outer situated side wall and the outer diameter of the inner situated side wall for a given pole ray and, on the other hand, to the diameter of the circulatory movement.
From these considerations it follows that for a certain machine size there is present a greater compressibility factor or expansion factor at the expense of the conveyed quantity or vice-versa. This is so because for a large compressibility factor or expansion factor the ratio, or stated in a more simple manner, the difference between the mean diameter of the section of the spiral spanning 360.degree. and following the machine inlet and the mean diameter of the section of the spiral spanning 360.degree. preceding the machine outlet, must be chosen to be as large as possible. This difference becomes that much greater the smaller the width of the displacement compartment, i.e. the smaller the possible conveying capacity of the machine.
A state-of-the-art machine which satifies the above considerations has been disclosed, for instance, in FIGS. 14 to 16 of U.S. Pat. No. 801,182. With this machine the spiral spans both the displacement compartment as well as also the displacement element approximately 4 times 360.degree.. The compressibility factor or expansion factor for this machine is estimated to amount to 3. In order to obtain such a conveying compartment between the displacement element and a side wall of the displacement compartment must extend from the machine inlet 4 times completely about the pole of the spiral before it reaches the machine outlet. With the exception of the first complete circulatory movement following the machine inlet and the last complete circulatory movement preceding the machine outlet, the conveyed medium of this prior art machine thus must move through an unnecessarily long path, increasing the spatial requirements of the machine or, with the same size machine, impairing the conveying capacity thereof.